This invention relates to a cover for an air bag and to a method for producing the same.
Air bag safety systems are commonly fitted in new motor vehicles.
These systems comprise a cushion which is normally stored in a compartment in the steering column and which can be rapidly inflated. There may also be a passenger side air bag located in the dashboard or fascia. Airbags may also be stowed in the vehicle doors.
In the event of a crash, sensors detect the vehicle rapidly decelerating and cause the air bag to be rapidly inflated into the vehicle interior to cushion the occupants as they are thrown forward or sideways depending on the direction of impact.
The compartment or compartments containing the air bags are closed by a cover to protect the air bags from accidental or malicious tampering, and to prevent any object entering the airbag compartment which could become a dangerous projectile on deployment of the bag. It is important that the cover is able to withstand knocks and jolts inflicted upon it. In particular, it must comply with the relevant European or U.S. standard governing passenger impact. It is a further requirement that the cover must not shatter or become detached when forced open, as it may in such case be projected towards and injure the vehicle occupants.
One approach to the above problem is illustrated in EP-A-0363986 which describes an airbag housing in which the airbag deployment aperture is closed by a pair of moulded plastic doors. These doors are attached at one side to the fascia or dashboard by hinging portions, and are joined to each other by a frangible bridging piece. A disadvantage of this arrangement is that the doors, being separately constructed from the fascia, are visible from the interior of the vehicle.
A second approach to the problem is disclosed in WO 95/24328 in which a line of mechanical weakness is introduced from the rear of the cover. Such an arrangement allows the designer to construct the fascia to give the appearance of continuity. However, there are difficulties in implementing such an approach. For example, due to inherent limitations in the manufacturing process, the trim piece varies in thickness over its entire extent, making it difficult to provide the accurate and consistent line of weakness required for the trim piece to rupture reliably. Even if a conventional tool could be controlled well enough to remove a constant thickness of material to provide a line of weakness, that remaining is not of constant depth.
Such non-consistent thickness means that the trim piece will not tear reliably or evenly, so hindering the successful deployment of the air bag.
A further disadvantage of a moulded cover as described in EP-A-0363986 is that it must be subjected to a much greater force than a cover formed by cutting before the frangible portion ruptures and the doors open. In some cases, the manufacturing process causes the finish of the weakened portion of the trim piece to degrade over time, when viewed from the face side of the dashboard.
It is now proposed to produce an integrated air bag cover by using a laser beam to provide a line of mechanical weakness in a motor vehicle trim piece or panel. The cover is produced by a method comprising the steps of:
i) scoring the panel from its underside with a laser beam to form a line of mechanical weakness, and
ii) maintaining a substantially constant thickness of intact material within said score line by monitoring the depth of penetration of the laser beam into the panel, or the thickness of material remaining in the score-line.
The trim piece may be a three layer laminate, but may equally be of different construction The trim piece may be formed in accordance with the method made known in European Patents Nos. 0466181 and 0480456, granted to Sumimoto Chemical Company, the disclosures of which are hereby incorporated by reference.
The trim piece may also be manufactured by a vacuum adhesive process. In such a process, a pre-vacuum formed pad (e.g. of PVC/thermoplastic polyolefin and polypropylene foam) is adhesively bonded to a polypropylene injection moulded part, and the combined laminate is subject to vacuum moulding to finally form a moulded laminate.
Preferably, the trim piece comprises a rigid substrate, a resilient intermediate layer, and a decorative foil, forming the outer layer.
More preferably, the trim piece comprises a thermoplastic polyalkylene substrate, a low density polyalkylene foam, and a polymer foil.
Conveniently, the substrate is polypropylene (hereinafter PP), the intermediate layer is foamed polypropylene, and the foil is an acrylonitrile butadiene-styrene/polyvinyl chloride copolymer (ABS/PVC COMPOSITE) or thermoplastic polyolefin (TPO). It is also envisaged that trim pieces may be made from layers of a single substance, such as a TPO, in successive layers of solid, foamed and foil materials so that the trim pieces can be readily recycled at the end of their useful lifetime.
The laser is preferably a carbon dioxide laser. Alternatively, an Nd:YAG, argon gas, diode, excimer or solid state laser may be used. Preferably, the laser emits a pulsed beam
Conveniently, the thickness of the remaining uncut material is determined using a sensor located at the upper or face side of the panel, the panel or trim piece being disposed between the laser and the sensor. The laser scores the trim piece more accurately than mechanical cutting tools.
To determine the depth of the uncut material, the laser beam is made to penetrate the trim piece completely and impinge on the sensor which then informs a controlling computer to shut off the beam.
A timing device or a pulse counter measures the time or number of pulses taken to penetrate the trim piece, allowing the controller to ascertain the depth of cut and hence the thickness of the intact material within the score line.
Theoretically, a single microperforation is all that is necessary to establish a reference point for the remainder of the cut along the score line. However, in order to maintain close control in case of variations in the material or initial thickness of the panel, it is preferred to provide a series of microperforations spaced along the line of weakness. Once the number of pulses required to just penetrate the panel is determined, the computer controller adjusts the number of pulses to less than this number so that a layer of intact material of desired constant thickness remains in the trough of the score line.
The score line formed by the laser need not be continuous. It may comprise a series of aligned holes separated by uncut material. The holes will generally be xe2x80x98blindxe2x80x99. In other words, they will not entirely penetrate the thickness of the cover or trim piece. As described above, the score line will preferably have one or more microperforations extending through the entire thickness of the laminate forming the cover, which are followed by a series of blind holes formed by the laser. In a typical laminate comprising a base layer, foamed intermediate layer and outer foil layer, the blind holes will generally penetrate the entire depth of the base layer. Commonly, they will also penetrate the entire thickness of the foam layer.
In order to vary the degree of mechanical weakness imparted by the lasercutting operation, the pitch, i.e. the distance between blind holes, may be varied so that if less weakening is required, e.g. at a hinge portion, or with a thinner laminate, the pitch can be increased.
The laser may also be used to etch a design or wording on the top surface of the trim piece. For example, the word xe2x80x9cAirbagxe2x80x9d may be etched.